The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
It is sometimes desirable to combine two polymeric materials into an article to take advantage of properties of each of the materials, such as an elastic material and an inelastic material or a thermoplastic material and a thermoset material. Lyden et al., U.S. Pat. Nos. 5,906,872, 5,843,268, 5,786,057, and 5,709,954 disclose chemically bonding a rubber and a plastic in making, for example, a footwear outsole, by molding the plastic into a first piece and then molding and curing the rubber in the presence of the molded plastic piece. It is important in this process for the melt temperature of the plastic to be higher than the rubber curing temperature to prevent deformation of the molded plastic insert piece during rubber molding and curing. There are many plastics unsuitable for this process due to lower melting or softening points than can withstand the rubber processing conditions.
Hernandez, US Patent Application Publication No. 2004/0000255 describes forming a foamed cushion pad from a mixture of SBS (styrene-butadiene-styrene copolymer), EVA (ethylene-vinyl acetate copolymer), filler, curing agent, and a blowing agent, then inserting the foam pad into a recess of the upper mold in forming an outsole. The outsole compound is 50-100 wt % SBS and 45-60 wt % EVA, filler, curing agent, and a processing oil. Peroxide is used as curing agent for both the cushion pad and the outsole.
Lorenzin, US Patent Application Publication No. 2004/0026820 describes a process for manufacturing a sole in which a vulcanized rubber (NBR) containing a small amount of an acrylic resin with hydroxyl functionality and a hydrocarbon resin is cured in a mold to form a tread sole. A two-component polyurethane is then injected and cured to form a mid-sole. The publication postulates “mating” because the rubber surface retains “many free bonds” after vulcanization, impliedly from the hydroxyl acrylic resin.
Finally, Krajcir, U.S. Pat. No. 6,007,748 discloses a method of making a molded laminate sole for footwear in which a first heat curable material is placed in a mold, a second polymerizable material is injected to fill the remainder of the mold cavity, and polymerization of the second material generates heat that cures the first material. The document discloses foamable, polymerizable polyurethane, synthetic rubber, and PVC as suitable for the second material and mentions vulcanizable rubber as the first material.
There remains a need for an effective way to assure good adhesion of a thermoplastic layer to a cured rubber layer in a composite of such layers, such as is used as a sole assembly for footwear, without the disadvantages of using adhesives or the limitations on selection of thermoplastics used as an insert in molding rubber compositions.